1. Field of the Invention
The present invention generally relates to a double mode modulator and, more particularly, to a double mode modulator for portable telephones which is adapted to be capable of realizing both digital and analog modulations.
2. Description of the Prior Art
Recently, portable telephones have been greatly developed from analog telephones using the conventional FM (Frequency Modulation) method to digital telephones using the digital modulation method.
Nevertheless, a number of users still have a portable telephone of AMPS (Advanced Mobile Phone Service) relying upon the conventional analog frequency modulation method and there exist areas where only analog method is supported for portable telephones. It is thus general that telephones which support both the conventional analog method and the digital method are widely used.
As regulated in the EIA/TIA (Electronic Industry Association/Telecommunications Industry Association) Interlim IS-95, the double mode method of portable telephones may support both analog method using the narrow-band frequency modulation and digital method using the QPSK (Quadrature Phase Shift Keying) technique. Herein, the analog method is to transmit the user's voice or data signals modulated by the frequency modulation method at a frequency whose bandwidth is 30 KHz, while the digital method is to convert an analog signal to digital information and then transmit/receive it after a separate signal processing step.
The most widely used, conventional frequency modulation method is realized with the PLL (Phase Locked Loop) circuit including a voltage-controlled oscillator (hereinafter, referred to as "VCO"). FIG. 1 is a diagram illustrating such a conventional frequency modulation method.
As illustrated in FIG. 1, PLL 100 includes a FD (Frequency Divider) 111, a PFD (Phase Frequency Detector) 112, a CP (Charge Pump) 113, a LF (Loop Filter) 114 and a VCO 115. The PLL 100 is a device for generating a frequency shift based on a signal input voltage, with the input terminal of the VCO 115 is also connected to a separate modulation input.
The frequency divider 111 includes a frequency divider 111a for dividing a frequency by M (M is a constant), and a frequency divider 111b for dividing a frequency by N (N is a constant). The phase frequency detector 112 detects a phase difference between the two input signals received from the frequency divider 111. The charge pump 113 transfers charges to the loop filter 114 as much as the phase difference which is the result of the operation of the phase frequency detector 112. The loop filter 114 determines the frequency response characteristics with respect to a control signal for the VCO 115 by use of an error signal that corresponds to the phase difference. This loop filter 114 plays a very important role in determining the frequency characteristics of the entire frequency synthesizer. The VCO 115 is an oscillator whose oscillating frequency changes with an applied voltage varied.
The characteristics of the entire VCO 115 in this case normally is determined by the open loop gain of the PLL 100 to a great extent. That is, where the open loop gain is greater than unity, this change of the output frequency is offset by a feedback of the PLL itself with respect to an input modulated signal, and accordingly, it decreases with respect to the entire input modulated signal.
Such a case is simply illustrated in FIGS. 2 and 3.
FIG. 2 is a response curve of the open loop gain with respect to the frequency of the PLL, which plays an important part in determining the frequency response of the PLL.
FIG. 3 shows the characteristics of output frequency shift with respect to input signals in modulation, wherein .omega.1 is the frequency whose entire loop gain is unity, and the frequency having the loop gain greater than unity will not be modulated.
In case such a structure as shown in FIG. 1 is used in the frequency modulation, a cut-off frequency for low-frequency wave of the output modulated waves depends upon the loop bandwidth of the entire PLL.
On the other hand, the loop bandwidth of the PLL is determined by various factors including the characteristics of the entire circuit constituting this frequency synthesizer, such as VCO gain, PFD gain and the time constant of the loop filter. Accordingly, a cut-off frequency for low-frequency wave with respect to input signals varies according to the values of the respective devices, and it is difficult to realize the circuit which is adapted to perform an additional operation for adjustment in order to compensate for such variations.
Furthermore, it might be better to increase the loop bandwidth in order to obtain a low phase noise and a short locking time, in which case a desired frequency modulation is hard to realize properly because the cut-off frequency becomes higher.
The use of both digital and analog circuits which are separately constructed in realizing a portable telephone causes the cost and size of the telephone to increase. It is thus very important to realize a circuitry that supports the digital and analog methods simultaneously with minimum circuits.
PLL and varactor are generally used in realization of a frequency modulation circuit. With this respect, the frequency modulation circuit is constructed to modulate analog signals having frequency components down to less than several hundreds of hertz in order to satisfy the frequency characteristics of AMPS signals. To embody such a construction, the loop bandwidth of the PLL has to be less than several hundreds of hertz, and the frequency characteristics are required to be precisely adjusted to meet those of AMPS signals. These characteristics are determined by the time constant of external components and need separate adjustment with deviation of time constant between the devices. Moreover, such a frequency modulation circuit is constructed with components which is difficult to integrate in an integrated circuit, so that it arises a problem of an increase in the cost, power consumption and size of the portable telephone.